Carlo Longino points out on his blog that Steve Jobs has said battery capacities and immature 3G chipsets that take too much power are the reason the iPhone doesn’t have any 3G capabilities. If you want to build a phone these days that is designed for only being used for a two minute voice call once or twice a week, using a 2G chipsets is surely the right thing to do as standby power consumption is definitely lower than what 3G chipsets can do today. However, the iPhone is a multimedia device and is built for being used throughout the day for a myriad of purposes. Therefore 3G chipset power consumption is the least of your worries.
Compared to what the processor, display, background illumination, camera and memory consume during intensive use during the day, the additional power required for a 3G chipset while in standby is not worth mentioning. Even without a lot of network use my battery on the N93 is flat in the evening when I heavily use my phone during the day for taking pictures, navigating (NokiaMaps), taking notes, checking and responding to eMails, playing games, reading documents, etc. etc. All of this requires little to no network interaction. On days without a lot of activity the battery is still almost full in the evening, despite having been attached to a 3G network all day long. So 3G chipset idle mode power consumption is definitely not an issue if the phone / multimedia device is used heavily during the day.
But once you use the cellular network it doesn’t really matter if you use a 2G or 3G network. In both cases the battery is flat after two to three hours if I use the phone together with a notebook to access the Internet. I don’t think the iPhone is designed to do this but the same is true for using the network with the built in browser.
I wonder if Apple’s decision not to include 3G has more to do with the fact that you can count the number of 3G or 3.5G capable mobile phones (not datacards) in the U.S. on one hand these days. Compare that to Europe where 3G in mobile phones are already mainstream technology. If Apple had been a European company it could well be their decision would have been different.